We study environmental effects on human reproduction, together with the basic biology that underlies such effects. We published data this year showing that among healthy women who are trying to conceive, conception occurs with intercourse during a six-day interval of the menstrual cycle. This interval ends on the day of ovulation, suggesting that human sperm can survive up to five days in the female tract, while the ovum is fertilizable for less than a day. Based on these data we have developed statistical methods to estimate the chance of conception on each day of the cycle, taking into account possible effects of reproductive toxins. Contrary to popular belief, we found no evidence that couples can influence the sex of their baby by timing their intercourse around ovulation. The development of new methods can help advance a field of research, especially an emerging area such as reproductive epidemiology. Some of our most influential contributions have been in the development of analytic tools. In collaboration with colleagues in the NIEHS Biostatistics Branch, we published a new method for analyzing data on birth defects obtained in population-based registries. This method can be used to detect the presence of environmental teratogens that cause more than one kind of birth defect. Also, we have proposed a new method for the analysis of birthweight that can be used to detect very large babies as well as very small ones who are born outside the usual bell- shaped distribution of birthweight. Finally, we have demonstrated (using data from a prospective study of pregnant women in Denmark) that ultrasound examination to determine gestational age can be biased in environmental studies. This can occur when the fetus' growth (used by ultrasonographers to determine gestational age) is slowed by environmental or genetic factors, thus confusing the inference of gestational age.